In cognitive radio systems, the main requirement of the architectures is the ability to detect the presence of the primary user with fast speed and precise accuracy. In a cognitive radio system, signal detection mechanism is required for spectrum sensing in order to identify occupied and vacant channels and hence to detect the availability of the vacant spectra. To achieve this we have simulated the energy detection technique for cognitive radio system out of the three deferent techniques. In this paper we have emphasis on sensing the vacant channel for FM band using energy detection method and then the simulation results are ratified.